I've done a bunch of reading in this thread, and I'm a noob. Can someone explain what overlap means?

"Overlap" refers to the period of time, near the end of the exhaust stroke and the beginning of the intake stroke, when the intake and exhaust valves are open at the same time. For the exhaust valves, this is right before they close. For the intake valves, this is right after they have opened.

It might be counter-intuitive at first, but a carefully tuned amount of overlap can yield more power than no overlap at all. The lumpy idle that muscle-cars have with big cams is a side-effect of a relatively large amount of overlap. At higher RPM it's a good thing, but at idle it behaves like your intuition would lead you to expect - the motor pulls in some exhaust during the intake stroke, and combustion suffers.

With adjustable valve timing, we can tune the overlap to occur only in the conditions where it's actually helpful, so we get the increased volumetric efficiency in the midrange, but without the lumpy idle.